(Click here to see the story as it appears in the August issue of Modern Casting.)

Due to its generous hydraulic basins, more than 90% of electricity in Quebec, Canada, is produced by water turbines. However, numerous sites cannot be economically, technically or environmentally developed using conventional hydropower technologies (i.e., hydroelectric dams). Novel low power water turbines are sought to suit shallow, high flow rivers; they would be particularly effective in remote areas where they could replace fuel powered generators.

Each propeller for a 15kW water turbine uses five cast aluminum blades fixed to its axle. The Centre de Métallurgie du Québec, Trois-Rivières, Québec, Canada, was tasked with providing the technology to produce cast blades, meeting stringent conditions on as-cast surface finish (≤250 rms) and geometry (±2 mm [0.08 in.]) of the theoretical envelope (Fig. 1). Plus, 140MPa minimum yield strength and 2% elongation were required in the highest stressed parts of the blade.

Aluminum alloy A356 (AlSi7Mg04) was selected for its availability and excellent castability. It also exhibits good corrosion resistance when immersed in fresh river water. As for the casting process, the Centre de Métallurgie du Québec examined two methods for production—sand casting using traditional gating and sand casting with the direct pour method. If the metallurgical properties were found to be similar for the two filling procedures, direct pour technology would make the molding much easier in a smaller flask while increasing the yield and eliminating finishing costs.

In their study, the researchers compared the optimal operational parameters, such as pouring temperature and pouring time, for both methods. Metallurgical properties, including dendrite fineness (secondary dentrite arm spacing) and microporosity along with tensile properties, were measured at 13 locations.

Traditional Gating vs. Direct Pour Mold Filling

The wedge-shaped aluminum blade is a perfect geometry for casting, as solidification smoothly progresses from the thin end of the part to the thick extremity, providing self-feeding as long as a riser is placed at the hot end of the blade. The traditional rigging is shown in Fig. 2. It included a sprue, sprue well with a filter at the parting line, two runners and six gates corresponding to an unpressurized gating ratio of 1:2:4 with a resulting filling time of 25 seconds. The yield was 70%.

The cup of a direct pour system combines a ceramic foam filter held inside an insulating ceramic sleeve (see inset in Fig. 3). The assembly acts as a pouring cup, filter and riser, allowing the metal to be poured directly into the mold cavity, eliminating the traditional sprue-runner-gate system. It considerably simplifies the molding process and promotes directional solidification since the hottest metal is poured into the riser. Direct pour is particularly suited to the blade because the liquid metal drop under the filter is moderate (2 in. [50 mm]).

Comparisons

The pouring temperature was adjusted so the predicted temperature of the liquid metal front during filling would never reach a temperature below the liquidus temperature of aluminum A356 (1,135F [613C]). A pouring temperature of 1,364F (740C) with a 25-second fill time was set for the conventional gating, and a corresponding temperature of 1,274F (690C) with a 10-second fill time was set for the direct pour arrangement.

Filling with the conventional gating is shown in Fig. 4; a corresponding sketch for the direct pour filling is shown in Fig. 5. These two modeled fillings show conventional gating loses more superheat during filling due to heat losses in the runners and gates and due to its slower pouring rate.

The direct poured blade solidified slightly faster because of the lower pouring temperature. The thin edge of the blades solidified in about two minutes, while the thick end of the casting solidified in 20 minutes.

Microstructures of the castings were compared at locations L, C and E (indicated in Fig. 6). Dendrite arm spacing was virtually the same at the three locations; however:

In the thick section (L) of the casting, the level of microporosity is less in the conventionally gated blade (0.44% vs. 0.88%).

In the thin section of the casting, the level of microporosity is less in the direct poured blade (0.19% vs. 0.25%).

No significant differences in the metallurgical quality of the gated and direct poured blades were noticed in spite of the much lower pouring temperature used with the direct pour technology.

The blades were given a standard T6 heat treatment consisting of solutionizing for 12 hours at 1,000F (538C), followed by quenching in 149F (65C) water and aging for six hours at 320F (160C).

Thirteen tensile test bars were excised from the blades at locations shown in Fig. 7.

The yield strength, ultimate tensile strength and elongation at break appear in Table 1, along with the Quality Index (Q), which represents the metallurgical quality of the alloy.

The yield strength does not vary much around a value of 200 MPa. However, the ultimate tensile strength and elongation are higher in the thinner parts of the blade where the solidification time is lower. The quality index is less than 316 MPa, the minimum value of Q required for the ASTM B26 standard tensile specimen. This is explained by the fact that the 0.5-in. diameter standard specimen solidifies in about one minute while the solidification time in the blade varies from two to 20 minutes.

Heat Treatment and Modified Heat Treatment

Distortion from the quenching process was evaluated by measuring the shift at certain coordinates. This distortion should be reduced as much as possible. Since the mechanical properties in the blades exceeded the requirements (YS ≥140 MPa, El ≥ 2%), the researchers reduced the solutionizing temperature from 1,000F (538C) to 842F (450C).

The modified heat treatment reduced distortion considerably; however, it had to be verified that the mechanical properties still exceeded the minimum values specified.

After the modified heat treatment was applied to a blade, the quality index dropped by about 50 MPa, but the yield strength and elongation remained above the minimum required, particularly the yield strength (Table 2). The yield strength varied from 154 to 170 MPa, while the minimum required was 140 MPa. These properties by far exceeded the yield strength of an as-cast blade as previously determined by mechanical testing. The modified mild heat treatment more than doubled the yield strength of the as-cast blade.

Side Fill Direct Pour

In order to avoid a rough finish on the blade’s surface, the direct pour cup was modified to fill the cavity by the side rather than from the top (Fig. 8). This arrangement avoided the grinding of the blade surface at the cup-casting connection and provided a less turbulent liquid metal flow into the mold cavity. The surfaces receiving the hydro-kinetic energy were smooth, as-cast shapes providing optimum hydrodynamic efficiency for the water turbine (Fig. 9).

The direct pour filter cup technology studied on the blade highlighted the main advantages of direct pour:

1. Lower metal pouring temperature—1,274F (690C) vs 1,364F (740C).2. Simpler molding operation and a reduction of 25% in sand usage.3. Lower finishing costs and a near net shape as-cast product, especially when using filling from the side.4. Improved yield, from 70% to 85%.5. Reduced distortion in the as-cast condition as the fast-cooling runners and gates pull on the solidifying casting when the conventional rigging is used.

This article is based on paper 14-003 presented at the 2014 AFS Metalcasting Congress.